Photocatalytic Decarboxylative Hydroxylation of Carboxylic Acids

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Photocatalytic Decarboxylative Hydroxylation of Carboxylic Acids Driven by Visible Light and Using Molecular Oxygen Hai-Tao Song, Wei Ding, Quan-Quan Zhou, Jing Liu, Liang-Qiu Lu, and Wen-Jing Xiao J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01360 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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The Journal of Organic Chemistry

Photocatalytic Decarboxylative Hydroxylation of Carboxylic Acids Driven by Visible Light and Using Molecular Oxygen Hai-Tao Song, Wei Ding, Quan-Quan Zhou, Jing Liu, Liang-Qiu Lu* and Wen-Jing Xiao

Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan, Hubei 430079, China *Email: [email protected]

Abstract: This paper discloses the first example of photocatalytic direct decarboxylative hydroxylation of carboxylic acids. It enables the conversion of a variety of readily available carboxylic acids to alcohols in moderate to high yields. This unprecedented protocol is accomplished under extremely mild reaction conditions using molecular oxygen (O2) as a green oxidant and using visible light as a driving force.

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Introduction Decarboxylative hydroxylation is an important transformation in organisms and synthetic chemistry. In Nature, this process is catalysed by various hydroxylases in a highly efficient and specific way (Scheme 1a).1 For example, salicylate hydroxylase (SALH) which was isolated from Pseudomonas putida S-1 can selectively break the C-CO2H bond of a carboxylic acid and forge a new C-O bond in the conversion of salicylate to catechol, accompanied by the release of CO2.1d In contrast to enzymatic methods using molecular oxygen (O2) as green oxidant, many chemical methods for decarboxylative hydroxylations require the use of stoichiometric high-valent metal reagents as oxidants, such as Pb(IV), Mn(III), Tl(III) and Ce(IV), which result in much metal waste (Scheme 1b).2,3 In addition, Barton and coworkers developed an indirect method for the decarboxylative hydroxylation of carboxylic acids, in which corresponding thiohydroxamate esters need to be preformed (Scheme 1c).4,5 Though being favorable due to its mild reaction condition, green oxidant and wide substrate scope, this protocol suffers from the two-step operation, the stability of thiohydroxamate esters and the utilization of toxic agents (e.g., Bu3SnH or t-BuSH).6 Thus, the search of new, direct and green decarboxylative hydroxylation reactions of carboxylic acids is still a significant task for synthetic chemists. Herein, we describe a visible-light photocatalytic decarboxylative hydroxylation of carboxylic acids, which can be conducted under metal catalyst-free conditions at room temperature and by using O2 as a favorable oxidant (Scheme 1d).


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Scheme 1. Decarboxylative Hydroxylation of Carboxylic Acids

In the past years, visible-light-induced decarboxylative transformations of carboxylic acids have received increasing research interests for its prominent advantages.7,8 First, carboxylic acids are readily available, highly stable and inexpensive feedstocks. Second, the cleavage of C-CO2H bonds allows the subsequent functionalizations in a site-specific way. Third, the application of visible light photocatalysis9 in decarboxylative transformations of carboxylic acids enables the reaction to proceed under extremely mild condition. Last year, our group10 developed a decarboxylative carbonylative alkynylation of carboxylic acids through visible light photoredox catalysis (Figure 1, left).8c Critical to this success is the capture of a radical by the small molecule CO. Therefore, we



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expect that the same radical might react with the small molecule O2 and the resulting intermediate could be further converted into an alcohol product (Figure 1, right).

Figure 1. Hypothesis: O2-capture of radicals for photocatalytic decarboxylative hydroxylation.

Results and Discussion To test the above hypothesis, we initially examined the feasibility of this photocatalytic decarboxylative hydroxylation reaction using 2,2-diphenylacetic acid 1a as the model substrate. After examining a wide array of reaction conditions,11 we determined that the alcohol 2a can be produced in a good yield (80%) in the presence of photocatalyst PC-I (Mes-AcrClO4, 2 mol%), K2HPO4 (1.5 equiv.) and O2 in CHCl3 under irradiation of 7W blue LEDs at room temperature (r.t.), followed by a reduction operation (Table 1, entry 1).11 The data in Table 1 illuminated the effect of a variety of parameters on the reaction efficiency. For example, results of control experiments showed that photocatalyst, visible light, base and molecular oxygen are essential for this reaction (entries 2–5). Replacing of PC-I with other commercially available ruthenium and iridium photocatalysts (i.e., PC-II~IV), which have been previously proved to be efficient photocatalysts for the visible-light-induced decarboxylative functionalization of carboxylic acids, did not give alcohol 2a


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(entries 6-8). Photocatalysts PC-II, PC-III and PC-IV possess lower oxidation potential [e.g. E1/2(PC*/PC-) = +0.66 V for PC-II; E1/2(PC*/PC-) = + 0.77 V for PC-III; E1/2(PC*/PC-) = +1.27 for PC-IV]9c than the Fukuzumi catalyst PC-I [E1/2(PC*/PC-) = +2.06].9g Obviously, PC-II and PC-III are difficult to oxidize the carboxylic ion 1a (E1/2ox = +1.07 for 1a)8j in this reaction. However, the reason why PC-IV [E1/2(PC*/PC-) = +1.27] is not an efficient photocatalyst for this transformation is not clear at current stage. In addition to K2HPO4, other inorganic and organic bases (e.g., Na2CO3 and 2,6-lutidine) were also suitable for this transformation with comparable reaction efficiency (entries 9 and 10). When DCE or CH3CN was employed as the reaction media to replace CHCl3, a notable decrease of yield was observed (entries 11 and 12).



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Table 1. Effect of Reaction Parameters on the Photocatalytic Decarboxylative Hydroxylationa

variation from the standard conditions

yield/%b

1

none

78 (80c)

2

no PC-I

0

3

no visible light

0

4

no K2HPO4

0

5

Ar, instead of O2

0

6

PC-II, instead of PC-I

0

7

PC-III, instead of PC-I

0

8

PC-IV, instead of PC-I

0

9

Na2CO3, instead of K2HPO4

75

10

2,6-lutidine, instead of K2HPO4

76

11

DCE, instead of CHCl3

53

12

CH3CN, instead of CHCl3

43

entry

a

Standard conditions: 1a (0.5 mmol), PC-I (0.01 mmol, 2 mol%), K2HPO4 (0.75 mmol, 1.5

equiv.), O2 balloon and CHCl3 (5.0 mL) at r.t. under the irradiation of 7 W blue LEDs for 36 h; then NaBH4 (1.0 mmol, 2.0 eq.) and MeOH (2 mL) were added and stirred for 0.5 h. b c

Determined by 1H NMR analysis using 1, 3, 5-trimethoxybenzene as an internal standard.

Isolated

yield.

Mes-AcrClO4:

9-mesityl-10-methylacridinium

1,2-dichloroethane.


perchlorate.

DCE:

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2(PF6)-

R2

R1

(PF6)t

N

N Ru

ClO4- N PC-I: Mes-AcrClO4

N

Bu

N

N

N

R1 R2

N Ir N

N

R1

R1

N PC-II

PC-III: R1 = R2 = H PC-IV: R1 = F, R2 = CF3 t Bu

With the optimum conditions in hand, we started to probe the generality of this photocatalytic decarboxylative hydroxylation. As highlighted in Table 2, a wide range of substituted benzylic carboxylic acids were suitable for the current reaction, producing corresponding benzyl alcohols in moderate to good yields. In addition to phenyl group, carboxylic acids bearing alkyl groups (i.e., methyl, ethyl, cyclohexyl, benzyl and allyl) at the benzylic position were proven feasible, providing the structurally varied alcohol products 2b-2f in 17-93% yields. However, the reaction with the ester-substituted substrate 2g only gave a complex mixture, presumably due to its instability under this condition. Moreover, phenyl acetic acids bearing electron-donating groups were also efficient substrates for this transformation (2j, 64% yield and 2k, 42% yield). Subquently, the electronic effect on this photocatalytic decarboxylative hydroxylation was investigated with 2-aryl propanoic acids as examples. We found that, incorporation of either electron-donating groups (Me and MeO) or electron-withdrawing groups (Br and Cl) on the benzene ring had no obvious impact on the reaction efficiency and the corresponding products 2l-2o were obtained in 69–75% yields. Notably, heteroaryl-substituted carboxylic acids, such as thiophene, can readily undergo this reaction to provide the alcohol product 2p in a moderate yield. Interestingly, drug molecules Ibuprofen, Naproxen and Carprofen which belong to the family of non-steroidal anti-inflammatory (NSAI) agents,12 can also undergo this decarboxylative hydroxylation process smoothly, affording the corresponding hydroxylation products in moderate yields (2q-2s).



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Table 2. Generality of the Photocatalytic Decarboxylative Hydroxylationa

a

Reaction conditions: 1 (0.5 mmol), PC-I (0.01 mmol, 2 mol%), K2HPO4 (0.75 mmol, 1.5 equiv.),

O2 balloon and CHCl3 (5.0 mL) at r.t. under the irradiation of 7 W blue LEDs; then NaBH4 (1.0


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mmol, 2.0 eq.) and MeOH (2 mL) were added and stirred for 0.5 h. Yields of isolated products. Beyond these benzylic carboxylic acids, other substrates were tested for this photocatalytic reaction. For example, when cyclohexanecarboxylic acid 1t and adamantane-1-carboxylic acid 1u were subjected to the standard conditions, to our delight, these reaction proceeded well, delivering the corresponding alcohols in moderate yields (eq. 1: 2t, 56% yield; eq. 2: 2u, 40% yield).

Following, we performed more decarboxylative hydroxylation reactions to demonstrate the utility of this method. Replacing the blue LEDs with the ambient sunlight, the reaction of carboxylic acid

1d

still

proceeded

very

well

under

the

photocatalytic

condition,

providing

cyclohexyl(phenyl)methanol product 2d in a shortened reaction time and excellent yield (Scheme 2a, 9 h, 98% yield). Additionally, a gram-scale decarboxylative hydroxylation reaction of substrate 1d was carried out under the standard condition, we were pleased to find that the desired product 2d can be afforded in good yield, albeit at a prolonged time (Scheme 2b, 1.14g, 75% yield).



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Scheme 2. Demonstration of Synthetic Utility

Despite the success of this photocatalytic system, there are some patterns of carboxylic acids that fail to proceed with the decarboxylic hydroxylation reaction. As exemplified in Figure 2, when branched acyclic fatty acid 1v, heteroatom-containing carboxylic acids 1x and 1y were used as substrates, only complex reaction mixtures were observed instead of the corresponding alcohol products. In the case of primary aliphatic acid 1w, no reaction occurs under the standard conditions. Presumably, the active radicals are not easy to form or apt to decompose before its capture by molecular oxygen.

Figure 2. Unsuccessful substrates for this photocatalytic decarboxylative hydroxylation reaction. Furthermore, two control experiments were carried out to shed light on the reaction mechanism. As indicated in the literature,2 the oxygen atom comes from H2O or the anion of metal salts (i.e.,


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AcO-) in the decarboxylative hydroxylations of carboxylic acids mediated by high valent metals. That is because the radicals generated from carboxylic acids through these oxidative decarboxylation processes are prone to be over-oxidized; the resulting carbon cation intermediates facilely react with nucleophilic species H2O or AcO- to deliver the final hydroxylation products. To rule out this possibility in our transformation, the 18O-labeling experiments were performed using carboxylic acid 1a as the substrate. When 18O-labeling oxygen was used, corresponding 18O-labeling alcohol 2a was isolated in 82% yield and definitively confirmed by MS (eq. 3); when 18O-labeling H2O (10 eq.) was added to the standard conditions, no 18O-labelled alcohol 2a was observed (eq. 4). According to these results, the pathway in which the radicals generated from carboxylic acids were captured by O2 was believed to be more favorable. Meanwhile, it is well known that DABCO can be used to quench the singlet oxygen.13 The model reaction, however, worked very well even with the addition of two equivalents of DABCO and affords product 2a in 85% yield (eq. 5). Therefore, the singlet oxygen pathway could be ruled out from the current reaction.



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Thus, a possible mechanism was proposed to illustrate this photocatalytic decarboxylative hydroxylation according to the experimental evidences and published literatures (Scheme 3).8d,14-16 Firstly, the carboxylate A [E1/2(1a-/1a•) = +1.07V vs. SCE] which is generated in situ from carboxylic acid 1 under the basic conditions is oxidized to afford carboxyl radical B by the excited state of photocatalyst (Acr•-Mes•+) [E1/2(PC-I*/ PC-I-) =+2.06V vs. SCE].14,15 Then, radical C which comes from the photooxidized product B through release of CO2 is captured by soluble O2 to deliver peroxyl radical D.16 Finally, the mixture of intermediates E and F (confirmed by MS)，which is generated from D16b is reduced in situ by sodium borohydride and the alcohol 2 is produced. In addition to the capture of the carbon radical C, O2 [E1/2(O2/O2-.) = +0.99V vs. SCE] is also believed to function as a oxidant to regenerate the ground state of photocatalyst (Acr+-Mes) from the reduced state of photocatalyst (Acr•-Mes) [E1/2(PC-I-/PC-I) = -0.57V vs. SCE].15d

Scheme 3. Proposed Mechanism


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Conclusions In summary, we have developed a novel and direct decarboxylative hydroxylation reaction of carboxylic acids through visible-light-induced photocatalysis. This protocol enables the conversion of a variety of readily available carboxylic acids to alcohols in moderate to high yields. It is also favorable for its green and mild reaction conditions, such as the use of metal-free catalyst, green oxidants (O2) and sustainable energy (visible light or sunlight). Experimental Section General Information. All reagents and catalysts were purchased from commercial suppliers and used without further purification. All the solvents were treated according to general methods before use. NMR spectra were recorded on NMR spectrometers (400/600 MHz) in CDCl3 or DMSO-d6. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, m = multiplet, br = broad signal), coupling constants (Hz) and integration. Mass spectra were measured on a MS spectrometer. IR spectra were recorded on an IR spectrophotometer. All characterized data of products are coordinate with reported literatures.17 General Procedure for Photocatalytic Decarboxylative Hydroxylations. Carboxylic acid 1 (0.5 mmol), Mes-AcrClO4 (2 mol%) and K2HPO4 (0.75 mmol, 1.5 eq.) and CHCl3 (5 mL) were added to 10 mL of Schlenk tube. After charged with O2 using a balloon, the reaction mixture was stirred under irradiation of 7W blue LEDs (400-500 nm; max. absorption: 432 nm) (distance app. 5 cm) at room temperature. When the reaction finished (monitored by TLC), NaBH4 (1.0 mmol, 2.0 eq.) and MeOH (2 mL) were added to the mixture and stirred for 30 min. Then, the reaction mixture was quenched with saturated aq. NH4Cl, and extracted with DCM for three time (3 x 10 mL). The combined organic layers were washed with brine and dried over anhydrous Na2SO4. Finally, the



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solvent was removed in vacuo and the crude reaction mixture was purified by flash chromatography on silica gel [silica: 200-300; eluent: petroleum ether/ethyl acetate (30:1 – 20:1)] to afford the pure product. Diphenylmethanol (2a). 36 h, white solid: 73.7 mg, yield: 80%, mp. 70 oC. 1H NMR (600 MHz, CDCl3) δ 7.38 (d, J = 6.6 Hz, 4H), 7.34 (t, J = 7.8 Hz, 4H), 7.30 (m, 2H), 5.85 (s, 1H), 2.22 (br, 1H). 13

C NMR (100 MHz, CDCl3) δ 143.7, 128.3, 127.4, 126.5, 76.0. IR (in KBr): 3588, 3332, 1591,

1395, 1181, 1113 cm-1. MS m/z (EI): calcd for [C13H12O]+: 184.2, found: 184.3. 1-Phenylethanol (2b). 24 h, colourless oil: 42.8 mg, yield: 70%. 1H NMR (600 MHz, CDCl3) δ 7.42 – 7.32 (m, 4H), 7.29 – 7.26 (m, 1H), 4.91 (dd, J = 6.6, 2.5 Hz, 1H), 1.72 (br, 3H), 1.50 (d, J = 6.4 Hz, 3H).

13

C NMR (100 MHz, CDCl3) δ 145.7, 128.4, 127.4, 125.3, 70.3, 25.1. IR (in KBr):

3620, 3339, 1593, 1400, 1115 cm-1. MS m/z (EI): calcd for [C8H10O]+: 122.1, found: 122.2. 1-Phenylpropan-1-ol (2c). 24 h, colourless oil: 42.9 mg, yield: 63%. 1H NMR (600 MHz, CDCl3) δ 7.40 – 7.25 (m, 5H), 4.61 (t, J = 6.7 Hz, 1H), 1.86 – 1.73 (m, 2H), 1.69 (br, 1H), 0.93 (q, J = 6.0 Hz, 3H).

13

C NMR (100 MHz, CDCl3) δ 144.5, 128.3, 127.4, 125.9, 76.0, 31.8, 10.1. IR (in KBr):

3646, 3444, 3326, 1590, 1397, 1113 cm-1. MS m/z (EI): calcd for [C9H12O]+: 136.1, found: 136.0. Cyclohexyl(phenyl)methanol (2d). 36 h, white solid: 88.5 mg, yield: 93%, mp. 53 oC. 1H NMR (600 MHz, CDCl3) δ 7.49 – 7.09 (m, 4H), 4.36 (d, J = 7.2 Hz, 1H), 1.98 (d, J = 13.0 Hz, 1H), 1.78 – 1.72 (m, 2H), 1.70 – 1.57 (m, 3H), 1.37 (d, J = 13.2 Hz, 1H), 1.24 – 1.21 (m, 1H), 1.14 – 1.10 (m, 2H), 1.09 – 1.01 (m, 1H), 0.97 – 0.90 (m, 1H). 13C NMR (100 MHz, CDCl3) δ 143.5, 128.1, 127.3, 126.6, 79.3, 44.8, 29.2, 28.8, 26.4, 26.0, 25.9. IR (in KBr): 3524, 3338, 2926, 1593, 1449, 1080 cm-1. MS m/z (EI): calcd for [C13H18O]+: 190.2, found: 190.2. 1,2-Diphenylethanol (2e). 36 h, white solid: 43.6 mg, yield: 44%. 1H NMR (600 MHz, CDCl3) δ


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7.36 (d, J = 6.5 Hz, 4H), 7.30 (q, J = 5.9, 4.3 Hz, 3H), 7.24 (d, J = 7.2 Hz, 1H), 7.21 (d, J = 7.4 Hz, 2H), 4.91 (dd, J = 8.7, 4.7 Hz, 1H), 3.07 – 2.97 (m, 2H), 1.91 (br, 1H). 13C NMR (100 MHz, CDCl3) δ 143.7, 138.0, 129.5, 128.5, 128.4, 127.6, 126.6, 125.9, 75.3, 46.0. IR (in KBr): 3658, 3316, 1707, 1495, 1369, 1197 cm-1. MS m/z (EI): calcd for [C14H15O]+: 199.2, found: 199.6. 1-Phenylbut-3-en-1-ol (2f). 72 h, pale yellow oil: 12.6 mg, yield: 17%. 1H NMR (600 MHz, CDCl3) δ 7.39 – 7.27 (m, 4H), 7.27 – 7.23 (m, 1H), 5.87 – 5.71 (m, 1H), 5.13 (m, J = 3.5, 1.4 Hz, 1H), 4.78 – 4.67 (m, 1H), 2.51 (m, J = 8.8, 7.5, 6.2, 1.2 Hz, 2H), 2.03 (br, 1H). 13C NMR (100 MHz, CDCl3) δ 143.5, 134.1, 128.0, 127.1, 125.5, 117.8, 73.2, 43.6. IR (in KBr): 3666, 3311, 1707, 1494, 1368, 1201, 1045 cm-1. MS m/z (EI): calcd for [C10H13O]+:149.2, found: 149.2. 1,2,3,4-Tetrahydronaphthalen-1-ol (2h). 48 h, colourless oil: 32.6 mg, yield: 44%. 1H NMR (600 MHz, CDCl3) δ 7.47 – 7.39 (m, 1H), 7.24 – 7.16 (m, 2H), 7.11 (d, J = 5.1 Hz, 1H), 4.79 (t, J = 4.7 Hz, 1H), 2.83 (m, J = 16.5, 5.5 Hz, 1H), 2.76 – 2.70 (m, 1H), 1.99 (m, J = 11.2, 4.0 Hz, 2H), 1.94 – 1.90 (m, 1H), 1.78 (m, J = 9.7, 6.8, 3.4 Hz, 1H), 1.70 (br, 1H). 13C NMR (100 MHz, CDCl3) δ 138.7, 137.1, 129.0, 128.6, 127.5, 126.1, 68.1, 32.2, 29.2, 18.7. IR (in KBr): 3662, 2938, 1695, 1486, 1372, 1036 cm-1. MS m/z (EI): calcd for [C10H12O]+: 148.2, found: 148.4. 9H-Fluoren-9-ol (2i). 72 h, white solid: 46.5 mg, yield: 51%, mp. 158 oC. 1H NMR (600 MHz, CDCl3) δ 7.66 (dd, J = 7.4, 4.4 Hz, 4H), 7.40 (t, J = 7.5 Hz, 2H), 7.33 (t, J = 7.5 Hz, 2H), 5.59 (s, 1H), 1.93 – 1.45 (br, 3H). 13C NMR (100 MHz, CDCl3) δ 145.5, 139.9, 129.0, 127.7, 125.0, 119.9, 75.1. IR (in KBr): 3662, 3310, 1711, 1500, 1291, 1190 cm-1. MS m/z (EI): calcd for [C13H10O]+: 182.2, found: 182.5. (3,4-Dimethoxyphenyl)methanol (2j). 24 h, pale yellow oil: 53.8 mg, yield: 64%. 1H NMR (600 MHz, CDCl3) δ 6.94 (s, 1H), 6.90 (d, J = 8.1 Hz, 1H), 6.87 – 6.84 (m, 1H), 4.63 (s, 2H), 3.90 (s, 3H),



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3.88 (s, 3H).

13

C NMR (100 MHz, CDCl3) δ 148.8, 148.3, 133.5, 119.2, 110.8, 110.2, 64.9, 55.7,

55.6. IR (in KBr): 3523, 3442, 3338, 1596, 1514, 1458, 1328, 1142 cm-1. MS m/z (EI): calcd for [C9H12O3]+: 168.2, found: 168.5. (4-(Dimethylamino)phenyl)methanol (2k). 35 h, pale yellow oil: 31.8 mg, yield: 42%. 1H NMR (600 MHz, CDCl3) δ 7.24 (d, J = 8.7 Hz, 2H), 6.73 (d, J = 8.3 Hz, 2H), 4.56 (s, 2H), 2.94 (s, 6H). 13

C NMR (100 MHz, CDCl3) δ 150.2, 128.9, 128.5, 112.6, 65.2, 40.6. IR (in KBr): 3645, 2878,

1678, 1523, 1348, 1163 cm-1. MS m/z (EI): calcd for [C9H13NO]+: 151.2, found: 151.5. 1-(p-Tolyl)ethanol (2l). 36 h, colourless oil: 51.1 mg, yield: 75%. 1H NMR (400 MHz, CDCl3) δ 7.30 – 7.25 (m, 2H), 7.17 (s, 2H), 4.89 – 4.84 (m, 1H), 2.34 (s, 3H), 1.71 (br, 1H), 1.48 (d, J = 6.3 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 142.9, 137.0, 129.1, 125.4, 70.1, 25.1, 21.1. IR (in KBr): 3622, 3330, 1592, 1392, 1115 cm-1. MS m/z (EI): calcd for [C9H12O]+: 136.2, found: 136.1. 1-(4-Methoxyphenyl)ethanol (2m). 48 h, colourless oil: 52.5 mg, yield: 69%. 1H NMR (600 MHz, CDCl3) δ 7.30 (d, J = 8.2 Hz, 2H), 6.88 (d, J = 8.1 Hz, 2H), 4.85 (q, J = 6.5 Hz, 1H), 3.80 (s, 3H), 1.80 (br, 1H), 1.48 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 159.0, 137.9, 126.6, 113.8, 70.0, 55.3, 25.0. IR (in KBr): 3652, 3328, 1598, 1512, 1244, 1173 cm-1. MS m/z (EI): calcd for [C9H12O2]+: 152.2, found: 152.2. 1-(4-Bromophenyl)ethanol (2n). 48 h, white solid: 73.4 mg, yield: 73%, mp. 40 oC. 1H NMR (600 MHz, CDCl3) δ 7.47 (d, J = 8.1 Hz, 2H), 7.25 (s, 2H), 4.88 (q, J = 6.5 Hz, 1H), 1.80 (br, 1H), 1.48 (d, J = 6.5 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 144.70, 131.50, 127.10, 121.10, 69.73, 25.21. IR (in KBr): 3663, 3310, 1711, 1489, 1201, 1066 cm-1. MS m/z (EI): calcd for [C8H9BrO]+: 201.1, found: 201.4. 1-(4-Chlorophenyl)ethanol (2o). 36 h, colourless oil: 54.8 mg, yield: 70%. 1H NMR (400 MHz,


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CDCl3) δ 7.37 – 7.28 (m, 4H), 5.05 – 4.69 (m, 1H), 1.76 (br, 2H), 1.48 (d, J = 6.0 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 143.9, 132.5, 128.1, 126.5, 69.3, 25.1. IR (in KBr): 3594, 3336, 1593, 1491, 1402, 1086 cm-1. MS m/z (EI): calcd for [C8H9ClO]+: 156.6, found: 156.2. 1-(Thiophen-2-yl)ethanol (2p). 36 h, colourless oil: 29.5 mg, yield: 46%. 1H NMR (600 MHz, CDCl3) δ 7.24 (d, J = 4.7 Hz, 1H), 7.01 – 6.95 (m, 2H), 5.14 (d, J = 6.9 Hz, 1H), 1.79 (br, 2H), 1.61 (d, J = 6.8 Hz, 3H).

13

C NMR (100 MHz, CDCl3) δ 149.8, 126.6, 124.4, 123.1, 66.2, 25.2. IR (in

KBr): 3647, 3329, 1592, 1389, 1119 cm-1. MS m/z (EI): calcd for [C6H8OS]+: 128.2, found: 128.0. 1-(4-Isobutylphenyl)ethanol (2q). 36 h, colourless oil: 49.9 mg, yield: 56%. 1H NMR (400 MHz, CDCl3) δ 7.25 (d, J = 8.0 Hz, 2H), 7.10 (d, J = 8.0 Hz, 2H), 4.86 (q, J = 6.5 Hz, 1H), 2.46 (d, J = 7.2 Hz, 2H), 1.85 (m, J = 13.5, 6.8 Hz, 1H), 1.49 (d, J = 6.5 Hz, 3H), 0.90 (d, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ 143.0, 141.0, 129.2, 125.2, 70.3, 45.1, 30.2, 25.0, 22.4. IR (in KBr): 3646, 3313, 3018, 1672, 1513, 1366,1008 cm-1. MS m/z (EI): calcd for [C12H18O]+: 178.3, found: 178.2. 1-(6-Methoxynaphthalen-2-yl)ethanol (2r). 36 h, white solid: 74.8 mg, yield: 74%, mp. 115oC. 1H NMR (400 MHz, CDCl3) δ 7.73 (t, J = 7.0 Hz, 3H), 7.48 (d, J = 8.6 Hz, 1H), 7.18 – 7.10 (m, 2H), 5.04 (q, J = 6.6 Hz, 1H), 3.92 (s, 3H), 1.69 (br, 2H), 1.57 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 157.5, 140.9, 133.9, 129.3, 128.6, 127.1, 124.3, 123.7, 118.9, 105.6, 70.4, 55.2, 25.0. IR (in KBr): 3627, 3523, 3334, 1594, 1392, 1116, 1025 cm-1. MS m/z (EI): calcd for [C13H14O2]+: 202.3, found: 202.5. 1-(6-Chloro-9H-carbazol-2-yl)ethanol (2s). 72 h, white solid: 62.7 mg, yield: 51%. 1H NMR (600 MHz, DMSO-d6) δ 11.28 (s, 1H), 8.10 (d, J = 2.1 Hz, 1H), 8.00 (d, J = 8.1 Hz, 1H), 7.41 (d, J = 8.6 Hz, 2H), 7.28 (m, J = 8.6, 2.1 Hz, 1H), 7.08 (d, J = 8.1 Hz, 1H), 5.18 (s, 1H), 4.85 – 4.77 (m, 1H), 1.33 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, DMSO-d6) δ 146.8, 141.1, 138.9, 125.4, 124.3, 123.3,



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120.8, 120.7, 120.1, 117.5, 112.8, 108.2, 69.2, 26.9. IR (in KBr): 3405, 2971, 1719, 1503, 1155 cm-1. MS m/z (EI): calcd for [C14H13ClNO]+: 246.7, found: 246.7. Cyclohexanol (2t). 24 h, colourless oil: 28.1 mg, yield: 56%. 1H NMR (600 MHz, CDCl3) δ 3.61 (d, J = 9.0 Hz, 1H), 1.89 (d, J = 10.1 Hz, 2H), 1.76 – 1.71 (m, 2H), 1.55 (br, J = 13.3 Hz, 1H), 1.28 (m, J = 10.8, 9.6 Hz, 5H), 1.17 (d, J = 11.5 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 70.2, 35.4, 25.4, 24.1. IR (in KBr): 3614, 3443, 1591, 1392, 1113 cm-1. MS m/z (EI): calcd for [C6H12O]+: 100.2, found: 100.3 1-Adamantanol (2u). 48 h, colourless solid: 30.4 mg, yield: 40%, mp. 245oC. 1H NMR (400 MHz, CDCl3) δ 2.14 (s, 3H), 1.71 (d, J = 2.8 Hz, 6H), 1.62 (q, J = 12.7 Hz, 6H).

13

C NMR (100 MHz,

CDCl3) δ 68.2, 45.3, 36.0, 30.7. IR (in KBr): 3666, 3310, 2911, 1504, 1349, 1113 cm-1. MS m/z (EI): calcd for [C10H16O]+: 152.1, found: 152.2. Acknowledgment We are grateful to the National Natural Science Foundation of China (NO. 21232003, 21202053, 21472058 and 21572074) and other financial supports (NO. 201422, CCNU15A02007 and 2015CFA033) for support of this research. Supporting Information Details for condition optimizations, copies of 1H NMR and

13

C NMR spectra of products 2,

references for known compounds and the mechanism study. This material is available free of charge via the Internet at http://pubs.acs.org.

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(6) Liu, C.; Wang, X.; Li, Z.; Cui, L.; Li, C. J. Am. Chem. Soc. 2015, 137, 9820. (7) For a selected review, see: Xuan, J.; Zhang, Z.-G.; Xiao, W.-J. Angew. Chem. Int. Ed. 2015, 54, 15632. (8) For recent examples, see: (a) Huang, H.; Zhang, G.; Chen, Y. Angew. Chem. Int. Ed. 2015, 54, 7872. (b) Tan, H.; Li, H.; Ji, W.; Wang, L. Angew. Chem. Int. Ed. 2015, 54, 8374. (c) Zhou, Q.-Q.; Guo, W.; Ding, W.; Wu, X.; Chen, X.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem. Int. Ed. 2015, 54, 11196. (d) Wang, G.-Z.; Shang, R.; Cheng, W.-M.; Fu, Y. Org. Lett. 2015, 17, 4830. (e) Ventre, S.; Petronijevi, F. R.; MacMillan, D. W. C. J. Am. Chem. Soc. 2015, 137, 5654. (f) Zhou, C.; Li, P.-H.; Zhu, X.-J.; Wang, L. Org. Lett. 2015, 17, 6198. (g) Vaillant, F. L.; Courant, T.; Waser, J. Angew. Chem. Int. Ed. 2015, 54, 11200. (h) Griﬃn, J. D.; Zeller, M. A.; Nicewicz, D. A. J. Am. Chem. Soc. 2015, 137, 11340. (i) Candish, L.; Pitzer, L.; Gomez-Suarez, A.; Glorius, F. Chem. Eur. J. 2016, 22, 4753. (j) Capaldo, L.; Buzzetti, L.; Merli, D.; Fagnoni, M.; Ravelli, D. J. Org. Chem. 2016, DOI: 10.1021/acs.joc.6b00984. (9) For select reviews on visible light photocatalysis, see: (a) Narayanam, J. M.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102. (b) Shi, L.; Xia, W. Chem. Soc. Rev. 2012, 41, 7687. (c) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (d) Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355. (e) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 985. (f) Ravelli, D.; Protti, S.; Fagnoni, M. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.5b00662. (g) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00057. (10) For recent works on visible light photocatalysis from our group, see: (a) Xuan, J.; Xia, X.-D.; Zeng, T.-T.; Feng, Z.-J.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem. Int. Ed. 2014, 53, 5653. (b) Xuan, J.; Feng, Z.-J.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Chem. Eur. J. 2014, 20, 3045. (c) Xuan,


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J.; Zeng, T.-T.; Feng, Z.-J.; Deng, Q.-H.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem. Int. Ed. 2015, 54, 1625. (d) Guo, W.; Lu, L.-Q.; Wang, Y.; Wang, Y.-N.; Chen, J.-R.; Xiao, W.-J. Angew. Chem. Int. Ed. 2015, 54, 2265. (e) Zeng, T.-T.; Xuan, J.; Ding, W.; Wang, K.; Lu, L.-Q.; Xiao, W.-J. Org. Lett. 2015, 17, 4070, and 8c. (11) See Supporting Information for details of condition optimization. (12) Rieu, J. P.; Boucherle, A.; Cousse, H.; Mouzin, G. Tetrahedron. 1986, 42, 4095. (13) Pan, Y.;Wang, S.; Kee, C. W.; Dubuisson, E.; Yang, Y.; Loh, K. P.; Tan, C.-H. Green Chem. 2011, 13, 3341. (14) (a) Ohkubo, K.; Suga, K.; Morikawa, K.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 12850. (b) Kotani, H.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126, 15999. (15) (a) Cassani, C.; Bergonzini, G.; Wallentin, C.-J. Org. Lett. 2014, 16, 4228. (b) Chinzei, T.; Miyazawa, K.; Yasu, Y.; Koike, T.; Akita, M. RSC Adv. 2015, 5, 21297. (c) Wu, X.-X.; Meng C.-N.; Yuan, X.-Q.; Jia, X.-T.; Qian, X.-H.; Ye, J.-X. Chem. Commun. 2015, 51, 11864. (d) Wang, K.; Meng, L.-G.; Zhang, Q.; Wang, L. Green Chem. 2016, 18, 2864. (16) (a) Su, Y.-J.; Zhang, L.-R.; Jiao, N. Org. Lett. 2011, 13, 2168. (b) Yi, H.; Bian, C.-L.; Hu, X.; Niu, L.-B.; Lei, A. Chem. Commun. 2015, 51, 14046. (c) Jung, J.; Kim, J.; Park, G.; You, Y.-M.; Cho, E. J. Adv. Synth. Catal. 2016, 358, 74. (d) Wang, H.-M.; Lu, Q.-Q.; Qian, C. H.; Liu, C.; Liu, W.; Chen, K.; Lei, A. Angew. Chem. Int. Ed. 2016, 55, 1094. (17) Please see the references of known compounds in the Supporting Information.


Photocatalytic Decarboxylative Hydroxylation of Carboxylic Acids

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